Field
The present disclosure relates to novel sound attenuating structures in which locally resonant sonic materials (LRSM) act as membrane-type acoustic metamaterials (MAMs). The MAMs are able to provide a shield or sound barrier against one or more particular frequency ranges as a sound attenuation panel. More particularly, the disclosure relates to active control or adjustment of such panels by electromagnetic, electrostatic or other means.
Background
Sound attenuation panels are described in U.S. Pat. No. 7,395,898, which discloses a rigid frame divided into a plurality of individual cells, a sheet of a flexible material, and a plurality of weights. Each weight is fixed to the sheet of flexible material such that each cell is provided with a respective weight and the frequency of the sound attenuated can be controlled by suitable selecting the mass of the weight. The flexible material may be any suitable soft material such as an elastomeric material like rubber, or another soft material such as nylon. The flexible material is ideally impermeable to air and without any perforations or holes; otherwise the sound attenuation effect is significantly reduced. The rigid frame may be made of a material such as aluminum or plastic. The function of the frame is for support and therefore the material chosen for the frame is not critical provided it is sufficiently rigid and preferably lightweight.
In the above configuration, a single panel may attenuate only a relatively narrow band of frequencies. A number of panels may be stacked together to form a composite structure so that each panel is formed with different weights and thus the resultant panel attenuates a different range of frequencies in order to increase the attenuation bandwidth.
It would be desirable if the individual cells could be adjusted in order to adjust the range of frequencies attenuated by the individual cells, and consequentially the range of frequencies of the panel could be adjusted.